In general, the so-called slidacs is used as voltage adjustment device for alternating current (AC). The slidacs operates as the transformer for voltage adjustment because constant input power is supplied and target output voltage can be outputted arbitrarily. Its voltage adjustment is possible but it needs monitoring by an operator, and also lacks function of regulation for output stabilization. Thus the slidacs is not appropriate for power source for load or equipment requiring a stable alternating current (AC) voltage.
In order to improve such problems, a special device for power stabilization such as an automatic voltage regulator has been developed and used.
An automatic voltage regulator is an apparatus to maintain the output voltage at a constant value, and the automatic voltage regulator is used dependently for an alternator, a DC generator, the rectifier, and etc., and the automatic voltage regulator is used as a standalone device.
Patent document No. 1 discloses a technology for the automatic voltage regulator. The automatic voltage regulator comprises:                a transformer which has a primary inductive coil inducing the AC voltage input from the power supply section to the secondary side according to turns ratio and a secondary side coil decreasing or increasing the voltage induced from primary coil and outputting to the load side,        a plurality of tabs equipped in the output terminal of the secondary side coil to supply the load side with the different voltage according to the difference of the secondary side coil turns,        a switch unit including a plurality of triac devices for switching one tab among the plurality of tabs equipped in the output terminal of the secondary side coil to supply the load side with the voltage decreased or increased by turns ratio set for the corresponding the tab, and        a control unit for performing switching control of the switch unit to supply constantly the corresponding voltage according to user's setting of the load-side supply voltage.        
In the prior patent as described above, in case the output is set to 220V, when the tab of the triac device is switched to ON state, the voltage of 20V is generated on the opposite windings. Thus, in case of the input of 220V, 200V is outputted from its configuration.
But, a high EMF (electromotive force) is generated in the winding of the unused tab, and a high voltage is induced in the triacs, which causes a problem, that is, a conduction of the triacs regardless of ON/OFF signal.
That is, the conventional cases have the disadvantages that cause the triac problem by inducing the high EMF in the other unused tabs.